The present invention relates to low styrene content resins which exhibit reduced emissions of volatile organic compounds (VOC) compared to resin systems containing higher levels of styrene. More particularly the invention relates to a process for preparing unsaturated polyester resins which contain low levels of styrene (typically  less than 35% by weight based on the total combined weight of the resin and the styrene).
Much of the work on low VOC unsaturated polyester systems has focused on the use of waxes as a means of reducing emission. During cure, waxes, which are initially dissolved or dispersed in the resin, form a thin film on the surface of the fabricated article. The film acts as a physical barrier preventing styrene from evaporating from the surface of the curing part. This reduces styrene emissions. Unfortunately, this waxy film substantially diminishes interlaminar adhesion, reducing the strength of molded articles made using a multilaminate construction. An alternative to the use of wax, is to reduce the molecular weight of the unsaturated polyesters. The lower molecular weight polyester requires the use of less styrene to maintain an appropriate working viscosity. One common way to reduce molecular weight in polyester synthesis is to increase the concentration of one of the reactants relative to the other. Another technique is to use a monofunctional group to cap growing chains. Dicyclopentadiene (DCPD) based resins are a good example of the latter technique. The DCPD replaces carboxylic acid end-groups increasing solubility of the polyester in styrene. However, the DCPD groups can undergo side-reactions leading to a broad molecular weight distribution. Polymers with a broad molecular weight distribution tend to be higher in viscosity and require more styrene. Adding more DCPD further lowers molecular weight and more side reactions occur. In addition, the DCPD based resins perform poorly in corrosive environments and their mechanical properties tend to be at the low end of what is common for unsaturated polyester resins. In general, the higher the DCPD content the lower the performance.
An alternative to capping with DCPD is capping with a low molecular weight alcohol as described in recent U.S. Pat. Nos. (6,107,446 and 6,222,005), the contents of which are expressly incorporated herein. The ""446 and ""005 patents describe a process for preparing low viscosity resins with a low acid value (AV) and hydroxyl value (HV) that can be used in laminating applications. The process in ""446 and ""005 requires that a substantial amount of an alcohol be reacted with maleic anhydride (0.5 to 1.0 moles of alcohol per mol of maleic anhydride) followed by reaction with a glycol. In the reaction with glycol, a majority of the alcohol is removed along with water in the distillate. The efficiency of alcohol incorporation into the final resin is about 25%. The alcohol can be reused after purification but that is an added step and expense. Less alcohol can be used but incorporation efficiency does not improve and the number of polar end-groups increases. In addition, some of the resins made using this process are susceptible to air inhibition during cure at the air-laminate interface. This gives laminates with a tacky surfacexe2x80x94an undesirable characteristic.
It has been found that adding DCPD to the alcohol capping process described in patents ""446 and ""005 reduces the amount of alcohol required and can increase the efficiency at which the alcohol that is used is incorporated into the polymer. As is shown in the examples the amount of ethanol required by the present process is decreased by up to 50% and ethanol retention is doubled when DCPD is added. The incorporation of even small amounts of DCPD (10-15 mol/100 mol maleic anhydride) improves surface cure such that laminates dry tack-free. The resins prepared by the process of this invention do not suffer from the deficiencies in performance seen with traditional low viscosity styrene resins.
In order to achieve a usable viscosity at low styrene levels, the process of this invention utilizes esterification or transesterification reactions where small alkyl groups from the reaction of a monohydric alcohol and dicyclopentadiene (DCPD) moieties are added to the end of the polyester chains. Both the alcohol and DCPD are non-polar chain-ends, commonly referred to as caps, and replace polar end-groups such as a carboxylic acid or glycol hydroxyl. This gives an unsaturated polyester resin or xe2x80x9cdicapxe2x80x9d resin with a lower styrene requirement. Less styrene in the laminating resin reduces VOC emissions when the resins of this invention are molded into articles of commerce using open-molding techniques.
The dicap resins are prepared by reacting a carboxylic acid having at least two carboxyl functional groups and containing ethylenic unsaturation, ie. containing Cxe2x95x90C bonds, its corresponding anhydride or a mixture of suitable acids/anhydrides, with a saturated monohydric alcohol or mixture of alcohols such as methanol or ethanol, DCPD and water. The DCPD and alcohol can be reacted with the carboxylic acid/anhydride and water in any order or simultaneously. The carboxylic acid or anhydride can be reacted first with the alcohol followed by addition of water and DCPD or all the components can be reacted together. Generally, one reactor can be used for the entire reaction. This is called the one-pot method. Alternatively, the reaction can be conducted by reacting the carboxylic acid or anhydride, water and DCPD in one vessel and reacting the alcohol and carboxylic acid or anhydride in a second vessel then combining the contents of the two vessels and adding a glycol or glycols to prepare the final dicap resin. This is called the two-pot method.
The preferred method depends on reactor sizes and the configuration of the manufacturing facility. In most manufacturing facilities the one-pot method will be preferred. Using either method the reaction between alcohol and carboxylic acid or anhydride and DCPD and water with carboxylic acid or anhydride is conducted with some form of agitation such as stirring at about 158-300xc2x0 F. at atmospheric conditions.
Additives commonly used in preparing unsaturated polyester resins can be used. These include inhibitors, catalysts, and the like. The progress of the reaction can be followed by measuring the acid value (ASTM D1639-90) of the mixture. After substantially all of the alcohol and DCPD have reacted with the carboxylic acid/anhydride (one-pot method) the intermediate is thought to be a mixture of carboxylic acid acid/anhydride, monoesters and diesters where the DCPD and alcohol comprise the alcohol portion of the ester. At this point, the second step, glycols are added and the mixture is heated to 355-430xc2x0 F. with some form of agitation such as stirring. Volatiles are removed, preferably by distillation and the acid value (ASTM D1639-90) and viscosity (ASTM D1545-89) of the mixture are monitored until the desired end-point is reached. In addition the reaction with the glycols can be carried out in the presence of oils containing ethylenic unsaturation such as soybean oil. The reaction mixture is cooled and styrene is added to give the desired laminating resins. Inhibitors can be added to the styrene for extending storage stability of the resin.
Examples of unsaturated carboxylic acids and corresponding anhydrides useful in the invention include maleic acid, fumaric acid, itaconic acid and maleic anhydride. In addition other acids, anhydrides or esters of the acids can be added to modify the chemical composition. Examples of such acids and anhydrides include phthalic acid, isophthalic acid, terephthalic acid, tetrahydrophthalic anhydride, phthalic anhydride, nadic anhydride, methylnadic anhydride, hexahydrophthalic anhydride, dimethyl terephthalate and the like. Maleic acid and maleic anhydride are preferred.
Examples of saturated monohydric alcohols are those alcohols having a boiling point of less than about 300xc2x0 F. at standard temperature and pressure include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol and the like. Primary alcohols such as methanol and ethanol are preferred.
Dicyclopentadiene is used in the process of the invention as a capping agent. There are different grades of DCPD. Low grade DCPD typically has greater than 0.1% by weight of C-5 trimer. Polyester grade DCPD typically has less that 0.1% by weight C-5 trimer. Preferably, polyester grade DCPD is used. Polyester grade DCPD is available from Equistar.
A wide variety of polyols can be used in the process of the invention. Included would be common diols such as ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-propanediol, 1,4-butanediol, 2,2-dimethyl-1,3-propanediol, 2-methyl-1,3-propanediol, glycol ethers such as diethylene glycol and dipropylene glycol, and polyoxyalkylene glycols like polyoxyethylene glycol and polyoxypropylene glycol. Triols and higher functional polyols such as glycerol, trimethylol propane and oxyalkylated adducts thereof can also be used. Preferably, the polyols are aliphatic or alicyclic and optionally contain Cxe2x80x94Oxe2x80x94C linkages.
Examples of oils containing unsaturation include castor oil, peanut oil, linseed oil, safflower oil, olive oil, cotton oil, rapeseed oil, soybean oil and tung oil. In addition, fatty acids could be used instead of the oil. An example would be ricinoleac acid instead of castor oil. Modified oils such as epoxidized soybean oil may also be used. The use of soybean oil is preferred. Up to 45 wt % of the oil based on the total weight of all components charged minus distillate collected can be used. Preferably from 5 wt % to 45 wt % of oil is used. More preferably between 10 wt % and 30 wt % of oil is used in the process.
Other materials commonly used in the synthesis of unsaturated polyester resins, such as solvents, isomerization and/or condensation catalyst, promoters, etc. can be used in the process of the invention. Examples of solvents are those commonly known in the art and include but are not limited to hexane, cyclohexane, benzene, toluene, xylene, and mixtures of solvents. Commonly used inhibitors include hydroquinone, p-benzoquinone, di-t-butylhydroquinone, t-butylcatechol, phenothiazine, and the like. Catalysts used to promote the condensation reaction include p-toluene sulfonic acid, methane sulfonic acid, zinc salts (e.g. acetate), organotin compounds (dibutyl tin oxide) and other materials known to those skilled in the art. Isomerization catalysts include organic amines such as morpholine and piperidine.
In the following examples several dicap resins were prepared by the process of the invention. The following abbreviations were used EG-ethylene glycol, DPG-dipropylene glycol, DCPD-dicyclopentadiene, AV-acid value ASTM D1639-90, HV-hydroxyl value ASTM E 222-94, TS-tensile strength (psi), TM-tensile modulus (ksi) and ELG-elongation (%) were measured using ASTM D638, HDT-heat deflection under load (Co) ASTMD648-97.
In general, the process of this invention is carried out by reacting at about 90-175xc2x0 F. maleic anhydride with ethanol. When this reaction is completed, water and DCPD are added and the temperature is raised to 260xc2x0 F. Cooling may be needed during this step. The reaction with DCPD can be followed by a change (drop) in AV. Alternatively, the maleic anhydride, ethanol and DCPD can be reacted in one step. Once the target AV is achieved (AV will depend on amount of maleic anhydride, water, ethanol and DCPD that is used) the desired glycols, oils, inhibitors and catalysts are added and the temperature is raised to 380-420xc2x0 F. Volatiles are removed by distillation. Condensation is continued until the target AV and the Gardner-Holt viscosity (in stokes, by ASTM D 1545-76, (parts resin/parts styrene)) are reached. The product resin is diluted with styrene containing inhibitors and the final AV, HV and the final viscosity (cps) are measured using a Brookfield viscometer available from Brookfield Engineering Laboratories, Inc., 11 Commerce Blvd., Middleboro Mass. 02346).